1. Technical Field
The present invention relates to an electro-photographic recording apparatus such as a printer, a facsimile or a copying machine that visualizes an image using colored particles such as a toner, and more particularly to a fixing device fixing a toner image on a recording medium.
2. Description of Related Art
A recording apparatus using the electro-photographic method has a development process of visualizing the colored particles as the image on the surface of the recording medium, and a fixing process of fixing the visualized image of colored particles on the recording medium. For the colored particles, the powder called the toner intended for electrophotography is employed. The toner is molten by heating, and solidified by cooling. In the fixing process, the toner is heated and molten, and fixed on the recording medium, using the property of the toner.
The device for fixing the toner image on the recording medium relied on a method (roller fixing) in which a backup roller (counter roller) is pressed against a heat roller (heating roller) heated by a halogen heater disposed inside the roller by a coil spring or the like to form a nip part between the rollers, and the recording medium with the toner deposited is passed through the nip part to fix the toner by pressure and heat. Hereinafter, the heat roller is referred to as “HR”, and the backup roller is referred to as “BR”. Both the rollers HR and BR are called a fixing roller pair. Also, any one of the HR and BR is often called a fixing roller. The HR is heated, and the HR and BR are pressed together and rotated. By inserting the recording paper through this contact plane, the toner arranged as the image on the surface of the recording paper is fixed. Herein, at least one fixing roller may be heated. Also, when the toner image formed on the recording paper is inserted through the fixing roller pair, a carrying plane of the toner image is contacted with the heated fixing roller in inserting the toner image. In the fixing device, the toner may adhere to the fixing roller in fixing the toner image. This phenomenon is called an offset, and the toner adhering to the fixing roller is called an offset toner. If a large amount of offset toner arises, a problem arises that the offset toner re-transits to the recording medium to make no distinction from the recording medium, and cause a misprint. In the fixing device having a cleaner, the amount of toner to be wiped is increased, resulting in a problem that the exchange period of a cleaning member is shortened.
FIG. 1 is a graph showing an occurrence characteristic of the offset toner in a fixing device. Reference numeral 19 denotes a low temperature area, reference numeral 20 denotes a high temperature area, and reference numeral 21 denotes a non-offset band. The axis of abscissas represents the temperature of the heating roller, and the axis of ordinates represents the amount of offset toner. The amount of offset toner is greater in the low temperature area 19 and the high temperature area 20. The offset occurring in the low temperature area 19 is called a low temperature offset, and the offset occurring in the high temperature area 20 is called a high temperature offset. The area between the low temperature area 19 and the high temperature area 20 is called a non-offset band 21, where the amount of offset toner is small. In the fixing device, the temperature of the heating roller is usually set within the non-offset band 21 to reduce the amount of offset toner as much as possible.
The amount of offset toner is increased or decreased with the degree of load variation in an HR drive source, for example, an HR drive motor, at the time of fixing, as described in JP-A-6-19357. When the load variation is significant, the clear non-offset band 21 may not be found. This load variation occurs mainly due to a rush impact at the front end part of the recording paper in inserting the recording paper through the contact plane formed between the HR and BR, or an exhaust impact at the rear end part of the recording paper when exhausted.
The rush/exhaust impact is not large on the thin paper, in which the offset due to rush/exhaust impact is not problematical. If the thick paper is passed, a large rush/exhaust impact occurs, producing the offset at an impermissible level. To prevent the offset due to this rush/exhaust impact, a method for narrowing a range of impact coefficients for the pressing mechanism has been described in JP-A-6-19357.